Conventionally, there has been known a detection and ranging apparatus estimating a relative distance in a visual-line direction, a relative speed in a visual-line direction, and a direction (hereinafter referred to simply as a distance, a speed, a direction, or collectively as object specifications) between the apparatus and an object using radio waves (e.g., millimeter waves), acoustic waves, or light. According to such a detection and ranging apparatus, a probe signal is radiated toward a detection region covered by the apparatus. If an object exists in the region, the probe signal is reflected by the object to generate an echo signal. Then, the apparatus analyzes the echo signal to estimate the direction, the distance, the speed, and the like of the object. Of them, regarding the direction of the object, there is a detection and ranging apparatus, where a plurality of sensor elements are spaced at an appropriate distance apart, and estimation is made based on the phase of the echo signal received by each sensor element, the distance between the sensor elements, and the like.
In general, it is desirable that the detection and ranging apparatus having the above described functions should include a large number of sensor elements to detect the object with a good accuracy. However, for example, when the apparatus is to be mounted in a vehicle, the apparatus needs to solve a tradeoff between mounting a large number of sensors and keeping the apparatus compact. In light of this, a configuration for addressing the tradeoff has been proposed.
For example, holographic radar uses an oscillator to generate high-frequency signals which are transmitted as probe signals via a plurality of transmitting antennas. The probe signals are reflected by the objects to generate echo signals which are received by a plurality of receiving antennas (see Japanese Patent Laid-Open No. 2000-155171). Then, signal processing is performed on the obtained plurality of receiving signals to detect the objects. The above configuration uses time-division multiplexing to appropriately switch each of a plurality of transmitting antennas and a plurality of receiving antennas. Thereby, the same characteristics as those for the configuration of increasing the number of receiving antennas may be obtained.
Alternatively, for example, according to a radar apparatus having an N number of antenna elements, one of the N number of antenna elements disposed at the outermost periphery of an antenna array is alternately used as a transmitting antenna and the remaining N−1 number of antenna elements are used as the receiving antennas (see Japanese Patent Laid-Open No. 2006-98181). Thereby, the number of effective receiving antennas may increase to double the number of physical (i.e., actual) receiving antennas.
The performance and the size of the detection and ranging apparatus depend on the number of sensor elements (or antenna elements) and the arrangement. However, there has not been known a conventional method of implementing a design satisfying the required performance and/or size with a logical consistency. More specifically, conventionally, the number of transmitting sensors and receiving sensors and the arrangement thereof are determined by trial and error by considering each individual case separately. As a result, it is sometimes understood after an experimental device was made that the desired characteristics could not be achieved. It took a lot of effort to implement an appropriate design.
In addition, the aforementioned configuration includes a switch circuit for switching between the transmitting antennas and/or the receiving antennas. This switch circuit attenuates signals and thus may deteriorate estimation accuracy of the object specifications, namely, detection accuracy.